This article tries to answer a question about the greenhouse effect: "Greenhouse gases prevent the infrared rays from leaving the Earth's atmosphere, but why do they not prevent additional solar radiation from entering the atmosphere?"
The key is the different wavelength (or different frequency) of solar light and infrared light. Let's have a look at the greenhouse effect (see also the graph about radion transmitted by atmosphere below):
You could summarize this in saying the atmosphere does transmit 70 to 75% of solar radiation but only 15 to 30% of heat radiation.
This effect has always been there, it is not new at all. When the concentration of so-called greenhouse gases (carbon dioxide, Methane, NOx, etc.) is increased, the percentage of infrared radiation which is beeing transmitted through the atmosphere is reduced or in other words: When the concentration of greenhouse gases in the atmosphere is increased, more infrared radiation is reflected back to the Earth by the atmosphere. This leads to an increased temperature of the surface of the Earth. This increased effect due to higher concentration of greenhouse gases is normally called global warming or climate change.
This picture has been taken from Wikimedia (CC license by-nc-sa 2.5), see http://www.globalwarmingart.com/wiki/Image:Atmospheric_Transmission.png
Further information can also be found in Wikipedia.